Experimental and Numerical Investigation of a Complex Submerged Structure. Part II: Sound Radiation

The intention of the paper is to demonstrate the abilities and limitations of commercial Finite Element (FEM) and Boundary Element (BEM) packages, concerning the low frequency sound radiation of a complex submerged structure under free field conditions. Low frequency in this context means, that the considered frequencies are well below the coincidence frequency with Helmholtz numbers ka ≤ 1. The investigated object consists of two cylindrical shells with different radii, connected by a conical section and covered with hemispherical end caps. It contains ring stiffeners, flanges and rectangular frames. Part I of the paper covered the experimental and numerical modal analysis of the structure. The determined eigenmodes, achieved with a coupled FEM-BEM approach, served as velocity boundary conditions for the calculation of the sound radiation. A direct collocational boundary element scheme was used to compute two and three dimensional directivity diagrams of the particular eigenmodes of the investigated structure. The experiments were carried out on the research vessel PLANET in a Norwegian fjord with a water depth of more than 250 m. The submerged object was excited by an internal self mounted shaker at previously determined discrete eigenfrequencies of the structure below 300 Hz. Two dimensional directivity diagrams were measured by turning the object perpendicular to its axis of symmetry, while registrating the radiated sound by means of a vertical linear array at a distance of about 60 m. Thus, surface and bottom reflections of the radiated sound could be excluded. Experimental and numerical directivity diagrams will be shown for two particular frequencies.